I. Scope of the Invention
The present invention relates to the field of electronic instrumentation and AC power distribution grounding. More particularly, the present invention relates to a system for providing electrical safety and instrumentation quality AC power grounding by means of double insulation.
II. The Prior Art
In large volume experimental systems, such as large laboratories or in the case of field testing, test instrumentation is often located some distance from either the experiment being observed or a source of power. In many applications, such as mobile vans, it is virtually impossible to assure that the equipment grounds of the experiment and of the instrumentation AC power source are equipotential. Many pieces of equipment, whether test instrumentation or part of the experiment system, have their input signals referenced to their chassis, i.e., equipment ground. Thus, if high data quality is to be preserved, the experiment and instrumentation equipment grounds must both be at the same potential.
In typical prior art power distribution systems, power for a plurality of services, including both the experiment and instrumentation systems, is derived from a single power source. FIG. 1 shows such a power distribution system. The currents which flow in a neutral connection N between Bond 1 and Bond 3 produce potentials between the equipment grounds of the experiment and the instrumentation. If a shielded cable is connected between the experiment and the instrumentation, an AC power current will flow in the shield, thus producing AC power noise in the data. Such noise can be as high as several hundred millivolts upwards to several volts as compared to typical experimental data which is often as low as one millivolt. Such noise is therefore unacceptable.
One solution to eliminate the AC power noise is to break the neutral-to-equipment ground connection at Bond 3, allowing the instrumentation equipment ground to float. An external ground conductor, i.e. external to the power distribution wiring, may then be run between the instrumentation grounds. However, this results in a potentially life-threatening configuration. For example, the external ground wire is an uncontrolled conductor in that it may be accidentally broken independently of other AC power conductors. Further, since it is not considered an AC power conductor, there is no assurance that such an external ground wire can handle a full service fault.
If a hot-to-equipment ground fault occurs and the ground conductor has been disconnected, the full line potential will appear between the instrumentation equipment ground and that of the experiment. If a technician, under this condition, attempts to attach the ground conductor or data cable and comes in contact with both the instrumentation and experiment grounds, the resulting shock could be lethal. Even if the ground conductor is intact at the time of a fault, the conductor could be fused open if it is of insufficient size to handle the fault. In such a case, a technician attempting to attach a shielded data cable between the experiment and instrumentation could experience a lethal shock. For these reasons, running of a separate grounding conductor in lieu of the AC power equipment ground conductor is an unsafe practice.
This problem may be somewhat solved if an equipment ground conductor and all other AC power conductors are run from Bond 1 of the experimental volume to the instrumentation with no neutral-to-ground bond at the instrumentation. This configuration, termed a feeder, is shown in FIG. 2. The principal problem encountered with a feeder in instrumentation systems is due to the type of power line filters normally used. Such filters may cause currents on the order of several amperes to flow in the equipment ground, rather than in the neutral conductor. Such currents also cause a difference between the potentials of the experiment and instrumentation equipment grounds resulting in noise in the instrumentation and possibly an unsafe situation.
If an isolating transformer T is added to the feeder, as shown in FIG. 2, and a service derived at the instrumentation equipment ground, then filter displacement currents are allowed to flow in the neutral conductor with no current being present on the equipment ground conductor. This type of power distribution system is safe but, in practice, it is often impossible to obtain a feeder from the experiment AC power system in the manner shown in FIG. 2. Consequently, the equipment grounds of the experiment and instrumentation systems are rarely equipotential in actuality. While this problem may be solved by breaking the equipment ground path from the isolating transformer T to the instrumentation system and adding an external ground conductor from the instrumentation system to experimental volume ground, this reintroduces the hazards discussed above. For example, a primary-to-secondary fault in windings of the the isolating transformer T would produce a life-threatening situation if the external ground path were to be broken.
Thus, a problem exists in the art of providing both AC power to an instrumentation system and referencing that system to the ground of an experimental volume so that accurate test data may be achieved without threat of hazardous shock.